Field of the Invention
The invention relates to a device for switching inductive loads, in particular in a motor vehicle, including a series circuit which is located between a positive pole and a negative pole of a voltage source and has a load and a switch associated with the load, a free-running diode connected parallel to the load, and a guard circuit guarding against mispolarization of the voltage source.
A motor vehicle has inductive loads, such as magnet valves, with a current flow which must be controllable linearly (current flow.fwdarw.valve force.fwdarw.hydraulic pressure). As a rule, that is done by clocked switching of a load (such as a load L1 in FIG. 1) with a variable pulse width. Typically, integrated power semiconductors with MOSFET end stages (such as a switch Q1 in FIG. 1), which have a substrate diode that carries current from a source terminal to a drain terminal, are used for switching. A free-running diode (such as a diode D1 in FIG. 1) is connected parallel to the inductive load. As a rule, such a configuration is part of an engine or transmission control system (such as a controller ST outlined in dashed lines in FIG. 1), except for the inductive load itself.
One general problem in motor vehicle electronics is the possibility of mispolarization of the voltage source (such as a voltage source Bat shown in FIG. 1). In that case, the free-running diode and the substrate diode would be connected in the current flow direction to the voltage source, and the resultant current would destroy the free-running diode and the substrate diode. Current flow must therefore be prevented by suitable provisions if the voltage source is mispolarized, but the free-running current for the load must be preserved, and no components that reduce the voltage to be applied to the load can be allowed in the load current circuit.
For that reason, it is attractive to use a simple mispolarization guard diode, which is known per se, or another electronic switch, known from European Patent Application 0 436 778 A2, that becomes nonconductive if the voltage source becomes mispolarized, in the load current circuit.
A version including a main relay having an exciter coil, with a mispolarization guard diode, which is in series with the voltage source and can be excited through the vehicle ignition switch, is generally known. The disadvantages of that version are high cost, large structural volume, limited reliability, and the finite life of the contacts of the main relay.